1. Field of Invention
The present invention relates to a variable gain amplifier circuit.
2. Description of the Related Art
The Variable Gain Amplifier (VGA) is a critical baseband block of modern data communication transceivers. A VGA architecture was first proposed in the 1960s.
A representative example of a conventional VGA is shown in FIG. 1. The VGA 10 includes an input differential pair gain stage composed of transistors of 24 and 34 and resistors 40 and 42. It is biased by transistor 38. The amplification stage of the VGA of is comprised of two differential pairs. The first of these pairs consists of resistor 26 and transistors 20 and 22. Resistor 36 and transistors 30 and 32 form the second pair. The VGA additionally includes resistors 12 and 14 to reduce the current flowing through the differential pairs of the amplification stage. In operation, a differential input signal is applied across the differential input pair by connecting the positive input to the base of transistor 24 and the negative input to the base of transistor 34. Applying a differential signal across both differential pairs that comprise the amplification stage controls the variable gain of the VGA. The positive control signal is applied to the base of transistor 22 and transistor 30. The negative control signal is applied to the base of transistor 20 and transistor 32.
The conventional VGA architecture has been kept practically unchanged since its inception until today. It is compact and has both low noise and high input-to-output linearity. Its gain versus control voltage transfer function characteristics are linear when the gain is expressed in decibel units, which makes this VGA the preferred selection in high performance Automatic Gain Control (AGC) systems. It is the de-facto standard against which new VGA architectures are typically compared.
However, this architecture has some limitations and weaknesses. First, it has three stacked bipolar transistors and two resistors between the power supply and ground. Hence, it cannot be used with voltage supplies below ˜3 volts DC. Second, in order to obtain high linearity from input-to-output, the input differential pair stage transistors, transistors 24 and 34 as shown in FIG. 1, must be operating linearly. Thus, the current through the input differential pair stage transistors must be large. However, in order to minimize the noise of the circuit, only a small current should flow through the amplification stage containing transistors 20, 22, 30, and 32. These conflicting requirements have forced the addition of resistors 12 and 14 to the basic architecture in order to divert a significant part of the current away from the quad transistors 20, 22, 30, and 32. In other words, the addition of resistors 12 and 14 to the circuit introduces inefficiency because a significant amount of power is simply dissipated by resistors 12 and R8 14 serving no useful purpose other than reducing the current through the actual elements that implement the VGA functionality (e.g. transistors 20, 22, 30, and 32 and resistors 26 and 36).
For many years these limitations were considered irrelevant. The requirement that the minimum voltage supply be greater than approximately 3 volts DC was almost a non-issue because existing voltage supplies were commonly in the range of 5 to 12 volts DC. Further, the increased power consumption was also negligible because most circuits were ultimately connected to a line supply of current: e.g. a 110/220 volts AC power supply.
However, with the development of portable, battery-operated wireless transceivers (particularly remote wireless sensor networks), limitations on voltage supplies and power consumption are increasingly important. Specifically, the voltage supply has to be as low as possible and the current consumption has to be kept to a minimum in order to prolong the useful lifetime of battery-operated transceivers from a few hours to months or even years.
Since some emerging applications require increased efficiency that cannot be obtained by simply optimizing the conventional VGA, there is a need for new VGA architectures that can meet the requirements of such emerging applications. Accordingly, it is advantageous to use a VGA that provides increased efficiency by consuming minimal current and functioning with a low voltage supply.